Because its symptoms vary and its cause is unknown, GBS can be difficult to diagnose. If the symptoms occur uniformly across the body and progress rapidly, the diagnosis is easier.
Observation of the patient's symptoms and an evaluation of the medical history provide the basis for diagnosis of Guillain-Barre syndrome, although no single observation is suitable to make the diagnosis.
Three tests can confirm a diagnosis of Guillain-Barre syndrome.
Lumbar puncture (spinal tap)—The patient is given local anesthetic. Once the anesthetic has taken effect, a needle is inserted between two lower (lumbar) vertebrae and a sample of cerebrospinal fluid is drawn. An elevated level of protein without an increase in the number of white blood cells (WBCs) in the fluid is characteristic of GBS.
Electromyogram (EMG)—This is an effective diagnostic tool because it records muscle activity and can show the loss of individual nerve impulses due to the disease's characteristic slowing of nerve responses.
Nerve conduction velocity (NCV)—This test is performed with EMG, and together, they are often referred to as EMG/NCV studies. NCV records the speed at which signals travel along the nerves. These signals are characteristically slowed in GBS, although the findings may evolve over several weeks.